Water turbines, steam and gas turbines, windmills, centrifugal pumps and centrifugal compressors, and also propellers, are classified under the collective term turbomachines. Common to all these machines is the fact that they serve the purpose of extracting energy from a fluid in order to drive another machine as a result, or, vice versa, to supply energy to a fluid in order to increase its pressure.
In a turbomachine, the energy conversion is carried out indirectly and takes the path via the kinetic energy of the flow medium. In a turbine, for example, the flow medium flows through fixed stator blades, wherein the velocity and therefore the kinetic energy of the flow medium are increased at the expense of its pressure. As a result of the shape of the stator blades, a velocity component is created in the circumferential direction of the rotor wheel. The fluid or flow medium yields its kinetic energy to the rotor by the velocity value and the direction being altered during exposure of the passages, which are formed by the rotor blades, to throughflow. The rotor wheel is driven by means of the forces which are created in the process.
The rotating blades in a turbomachine are designed in a resonance-free manner for the widest possible range of operating volume changes, the blades may be subjected to excitation of vibrations which could lead to a failure of the blades if vibration resonances lead to excessively high mechanical stresses. Various devices have been developed in order to damp these vibrations. For example, it is known to couple blades to each other in order to damp vibrations as a result.
In DE 199 37 146 A1, a turbomachine is presented, in which permanent magnets are incorporated in the blade tip in order to couple adjacent turbine blades by means of magnetic forces.
EP 0 727 564 B1 discloses a turbomachine with turbine blades and a housing which is arranged around the turbine blade, wherein magnets consisting of rings are arranged in the housing on the circumference of the inner surface of the housing. The turbine blades have a conductive material on the tips, as a result of which vibrations can be reduced during a movement of these turbine blades towards the magnets.
In EP 1 596 037, a turbine blade arrangement is also disclosed, with which vibrations are to be reduced.
Vibrations of the blades are undesirable since they can lead to material fatigue of the blade and of the rotor steeple. Each per mil point of improved logarithmic damping decrement is desirable. Shrouded blades have for example an overall damping of 0.5% logarithmic decrement. A doubling of this value leads all round to a halving of the resonance amplitudes, which can mean that one mode less is to be determined. Also, the permissible speed range can be broadened as a result.
The available measures for damping vibrations have the disadvantage that they require a comparatively conditions. If the operating conditions change, for example as a result of large amount of installation space. This installation space, however, as a rule is not available. The high centrifugal forces which occur in turbomachines are a further limiting factor.
The vibration damping methods, which are induced by magnetic forces, such as in EP 0 727 564 B1, DE 199 37 146 A1 and EP 1 596 037 A2, have the disadvantage that the forces which are created as a result of eddy currents do not differentiate between a movement of the turbine blade tip in the principal movement and a disturbing vibrational movement. In other words, a movement of the blade in the rotational direction, i.e. in the circumferential direction, is influenced by the magnetic forces which give rise to eddy currents, which is undesirable. A vibrational movement which is not executed in the circumferential direction, for example in the axial direction, is to be damped by means of magnetic forces which give rise to eddy currents.
It would be desirable to have a device which damps vibrations of a blade, wherein the device does not have any influence upon the movement of the blade in the principal direction, i.e. in the circumferential direction.